The present invention relates to a thin film permanent magnet and a method for producing the thin film. Particularly, the present invention relates to a thin film permanent magnet suitably used for a micro-motor, a micro-actuator, a device for applying a biased magnetic field to a magneto-resistive device, a magnetic recording medium, or the like.
In the progress of miniaturization of various types of electrical apparatuses, the development of a micro-motor, a micro-actuator, and the like in which a thin film permanent magnet is employed is pursued. The size and the performance of a device of such a type depend on the magnetic properties of a thin film permanent magnet. For this reason, attention is focused, as a candidate material for the thin film permanent magnet on a Ndxe2x80x94Fexe2x80x94B based magnet material and a Smxe2x80x94Co based magnet material of which the maximum magnetic energy product is high, and the research and development on such materials are being extensively conducted. Among them, a special attention is focused on a tetragonal Nd2Fe14B compound constituting a main phase of the Ndxe2x80x94Fexe2x80x94B based magnet material as a high-performance thin film permanent magnet, because the saturation magnetization thereof is higher than the saturation magnetization of SmCo5 or Sm2CO17.
In the case of bulk-like permanent magnets, however, a Ndxe2x80x94Fexe2x80x94B based magnet of which the maximum energy product exceeds 400 kJ/m3 is manufactured, and commercially available. On the other hand, for a thin film permanent magnet primarily including the tetragonal Nd2Fe14B compound, it is difficult to improve both of the magnetization and the coercive force simultaneously, so that the thin film permanent magnet has not been put into practical use yet.
In the case of the thin film permanent magnet, one of the reasons which make the improvement of both of the magnetization and the coercive force difficult is that the magnetic anisotropy of the Ndxe2x80x94Fexe2x80x94B based magnet formed by thin film deposition technique is lower than that of a bulk-like permanent magnet manufactured by a method such as powder metallurgy, or other means.
In the case of the powder metallurgy, when a compact of magnetic powder is manufactured, highly anisotropic magnetic materials can be made by executing the orientation of magnetic powder in a magnetic field or by utilizing slide deformation.
As for the thin film deposition technique, an exemplary production of a perpendicularly magnetized film utilizing the anisotropic crystal growth is disclosed in F. J. Cadieu, et al, IEEE Trans. Magn. 22(1986) p.752, or the like, for example. It is considered, however, that the degree of technical perfection does not reach the level of the technology used for making anisotropic magnetic materials in the powder metallurgy.
As disclosed in K. D: Ayelsworth et al., Journal of Magnetism and Magnetic Materials 82(1989) p.48, an unintended impurity phase such as rare earth metal oxide or the like is often mixed or generated in a thin film having the tetragonal R2Fe14B compound as a main phase. This is one of the factors preventing the properties of a thin film permanent magnet from being improved.
Various trials have been performed for the purposes of improving the properties of a thin film permanent magnet. For example, Japanese Laid-Open Patent Publication No.7-6916 discloses a thin film permanent magnet in which a protection film is disposed on a rare earth alloy magnetic thin film. Japanese Laid-Open patent Publication No.9-219313 discloses a thin film permanent magnet in which protection films are disposed on and under a rare earth alloy magnetic thin film.
These protection films prevent the reaction between the rare earth alloy magnetic thin film and the air or a substrate, and exercise a function of preventing the magnetic properties of the rare earth alloy magnetic thin film from being deteriorated due to the reaction.
However, the objects of the above-mentioned prior arts are to suppress the reaction caused by the direct contact between the rare earth alloy magnetic film and the substrate or the air and to prevent the alteration of the magnetic film caused by the reaction. Thus, the metallurgical microstructure of the rare earth alloy magnetic film is not sufficiently controlled. Therefore, the coercive force is lower than a value expected from a crystal magnetic anisotropic energy originally included in the tetragonal R2Fe14B compound, and a sufficient residual magnetic flux density is not obtained.
In order to practically use the thin film permanent magnet, it is necessary to increase the energy product from the current value. For this purpose, it is necessary to control the metallurgical microstructure of the deposited rare earth alloy magnetic layer and further improve the magnetic anisotropy.
The present invention has been conducted in view of the above-described prior art. It is an object of the present invention to provide a high-performance thin film permanent magnet having both of high coercive force and high residual magnetic flux density by controlling the metallurgical microstructure. It is another object of the present invention to provide a rotating machine and a magnetic recording medium using such a thin film permanent magnet.
The thin film permanent magnet of the present invention is a thin film permanent magnet having a multilayer structure including four or more layers in which a refractory metal layer and a rare earth alloy magnetic layer are alternately deposited, and characterized in that the refractory metal layer is formed from at least one kind of material selected from a group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, and has a thickness of not less than 5 nm nor more than 50 nm, and the rare earth alloy magnetic layer has tetragonal R2Fe14B as a primary constituent phase (R is Nd and/or Pr), and has a thickness of not less than 50 nm nor more than 500 nm.
In a preferred embodiment, the rare earth alloy magnetic layer has magnetic anisotropy.
Preferably, a ratio (Br2/Br1) of a residual magnetic flux density (Br2) in a direction perpendicular to an in-plane direction to a residual magnetic flux density (Br1) in the in-plane direction of the rare earth alloy magnetic layer is 2 or more.
Preferably, the number of the rare earth alloy magnetic layers included in the multilayer structure is 3 or more.
Preferably, a ratio (tn/tm) of a total thickness (tn) of the refractory metal layers to a total thickness (tm) of the rare earth alloy magnetic layers included in the multilayer structure satisfies a condition of 0.01xe2x89xa6(tn/tm)xe2x89xa60.3.
In a preferred embodiment, a buffer layer is formed between a substrate for supporting the multilayer structure and the multilayer structure.
In a preferred embodiment, the buffer layer is formed from at least one kind of material selected from a group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.
In a preferred embodiment, a protection layer is formed as an uppermost layer of the multilayer structure.
The protection layer may be formed from at least one kind of material selected from a group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W.
The method for producing a thin film permanent magnet of the present invention includes the steps of: preparing a substrate formed from a material having a melting point of 300xc2x0 C. or more; and forming, on the substrate, a multilayer structure including four or more layers in which a refractory metal layer having a thickness of not less than 5 nm nor more than 50 nm formed from at least one kind of material selected from a group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W and a rare earth alloy magnetic layer having a thickness of not less than 50 nm nor more than 500 nm and having tetragonal R2Fe14B (R is Nd and/or Pr) as a primary constituent phase are alternately deposited.
In a preferred embodiment, in the step of forming the multilayer structure on the substrate, the rare earth alloy magnetic layer is formed while a temperature of the substrate is adjusted to be in the range of not less than 300xc2x0 C. nor more than 800xc2x0 C.
In the step of forming the multilayer structure on the substrate, the rare earth alloy magnetic layer may be formed while a temperature of the substrate is adjusted to be lower than 300xc2x0 C., and after the multilayer structure is formed on the substrate, the multilayer structure may be heated to temperatures of not less than 400xc2x0 C. nor more than 800xc2x0 C.
A preferred embodiment includes a step of applying a magnetic field to the multilayer structure during or after the formation of the multilayer structure.
A rotating machine according to the present invention is characterized by including any one of the above-mentioned thin film permanent magnets.
A magnetic recording medium according to the present invention is characterized by including any one of the above-mentioned thin films of permanent magnet.